1988 Volvo 740 Sugring

My problem is this, after 10-15 minutes the car starts to jerk/surge/sputter at idle and while driving, the tac jumps and quickly drops back to normal every second or two, it seems to stop while driving if I let off the throttle and then returns if givin more gas or goes back to idle. Only breifly would this symptom stop and the car would run smoothly ( maybe 5 seconds or more) and then of course the problem returns right away. Also it seems to have a small hesitation starting from a stop. This started to happen on the highway between the speeds of 50-60/65, ONLY, and would stop before or after those speeds. Also I may have a bad head gasket if that helps any, I assume that because I have a slight light brown oily color, but no other symptoms concerning that. New: distibutor, fuel pumps, fuel realy, ignition relay, air mass meter, air control valve. Plugs cap and rotor. I just set the timing back to 10 btdc was at about 13. Thanks.